In multi-frequency optical communications systems, such as wavelength-division multiplexing (WDM) and dense wavelength-division multiplexing (DWDM) optical communication systems, and in other multi-frequency optical systems, the need to select an optical signal of a given frequency from among a number of optical signals of different frequencies often exists. Optical filters are often used to perform such optical signal selection. However, conventional optical filters such as thin film filters or diffraction gratings generally select an optical signal of a given optical frequency or the optical signals in a range of optical frequencies. Other conventional filters such as fiber Bragg gratings only work in reflection mode and have very narrow tuning range. These properties limit the applications in which such filters can be used. Furthermore, tunable etalon filters suffer from multiple-order responses that also limit the applications in which they can be used. Thus, conventional optical filters usually do not have the ability to select a desired optical signal rapidly, reproducibly, over a wide frequency range, at low cost and with low energy consumption. Moreover, conventional optical filters typically work only in reflection mode and may have multiple-order responses.
Thus, what is needed is a tunable optical filter that is capable of changing the frequency of the optical signal that is selected, or rejected, rapidly, reproducibly and at low cost.